Scatter-free UV optical fluoride crystal elements for <200NM laser lithography and methods

Abstract

The invention provides a scatter-free below 200 nm wavelength transmitting optical fluoride lithography crystal for use with below 200 nm laser light. The invention includes making below 200 nm wavelength transmitting optical fluoride lithography crystals with a calcium fluoride feedstock in a low-chlorine graphite optical fluoride crystal crucible having a chlorine content concentration less than 0.3 ppm Cl by weight. The method includes melting calcium fluoride feedstock in the <0.3 ppm Cl graphite crucible to form a low-chlorine calcium fluoride crystal from the melt in the crucible to provide a grown calcium fluoride scatter-free crystal having a chlorine concentration less than 0.25 Cl by weight.

We claim:  
     
         1 . A scatter-free optical fluoride crystal for transmitting below 200 nm wavelengths comprising an optical fluoride crystal having a chlorine concentration of less 0.25 ppm Cl  
     
     
         2 . A scatter-free according to  claim 1 , wherein said optical fluoride crystal is selected from the group consisting of calcium fluoride, barium fluoride, magnesium fluoride, strontium fluoride and lithium fluoride, and mixtures thereof.  
     
     
         3 . The scatter-free crystal according to  claim 1 , wherein said optical fluoride crystal is a calcium fluoride crystal having a below 200 nm transmission >99%.  
     
     
         4 . The scatter-free crystal according to  claim 1 , wherein said optical fluoride crystal is a calcium fluoride crystal having a chlorine concentration of less than 0.2 ppm Cl and a 193 nm transmission >99%.  
     
     
         5 . The scatter-free crystal according to  claim 1 , wherein said optical fluoride crystal is a calcium fluoride crystal having a chlorine concentration of less than 0.2 ppm Cl and a 157 nm transmission >99%.  
     
     
         6 . The scatter-free crystal according to  claim 1 , wherein said optical fluoride crystal is a calcium fluoride crystal having a chlorine concentration of less than 0.2 ppm Cl and a 157 nm transmission >99%.  
     
     
         7 . The scatter-free crystal according to  claim 1 , wherein said optical fluoride crystal is a calcium fluoride crystal having a combined chlorine and sulfur concentration Cl+S of less than 0.3 ppm Cl and a transmission >98% in the 157-199 nm range.  
     
     
         8 . The scatter-free crystal according to  claim 7 , wherein Cl+S is less than 0.2 ppm and a 157 nm transmission >99%.  
     
     
         9  A method of making a scatter-free below 200 nm wavelength transmitting optical fluoride crystal, said method comprising: 
 providing a optical fluoride feedstock having a chlorine content less than 0.5 ppm Cl by weight,  
 providing crucible comprised of a low-chlorine graphite having a chlorine content less than 0.3 ppm Cl and placing said feedstock into said crucible,  
 melting said feedstock in said crucible to form a low-chlorine optical fluoride melt, and  
 growing an optical fluoride crystal from said melt,  
 wherein said grown optical fluoride crystal having a chlorine concentration less than 0.25 ppm Cl.  
 
     
     
         10 . The method according to  claim 9 , wherein the feedstock is selected from the group consisting of calcium fluoride, barium fluoride, magnesium fluoride, strontium fluoride and lithium fluoride, and mixtures of any of the foregoing.  
     
     
         11 . A method as claimed in 9 wherein the optical fluoride crystal is a calcium fluoride crystal, and said method includes transmitting a scatter inspection light into said grown optical fluoride crystal and inspecting the crystal for an observable level of scatter to provide a scatter-free optical fluoride lens blank with a chlorine concentration less than 0.2 ppm Cl by weight.  
     
     
         11 . A method as claimed in  9  wherein said feedstock chlorine content is ≦0.4 ppm.  
     
     
         12 . A method as claimed in  9  wherein said feedstock is calcium fluoride having a chlorine content is ≦0.25 ppm, said crucible graphite chlorine content is ≦0.25 ppm, and said grown calcium fluoride crystal has a chlorine concentration ≦0.2 ppm.  
     
     
         13 . A method as claimed in  12  wherein said grown calcium fluoride crystal has a 193 nm transmission >99%/cm.  
     
     
         14 . A method as claimed in  12  wherein said grown calcium fluoride crystal has a 157 nm transmission >97 %/o/cm.  
     
     
         15 . A method of making a scatter-free optical fluoride crystal for transmitting below 200 nm wavelengths, said method including: 
 providing a low-chlorine content optical fluoride feedstock having a chlorine content less than 0.5 ppm Cl,    providing an optical fluoride crystal crucible for containing an optical fluoride crystal, said crucible comprised of a purified graphite having a chlorine content less than 0.3 ppm Cl,    providing a controlled atmosphere optical fluoride crystal furnace for heating an optical fluoride crystal material,    loading said optical fluoride feedstock and said optical fluoride crystal crucible into said optical fluoride crystal furnace,    heating said feedstock into a low-chlorine melt and    growing an optical fluoride crystal from said melt to provide a grown scatter-free optical fluoride crystal having a chlorine concentration less than 0.25 ppm Cl and a below 200 nm transmission >99%/cm .    
     
     
         16 . A method as claimed in  claim 15 , wherein said optical fluoride feedstock is selected from the group consisting of calcium fluoride, barium fluoride, magnesium fluoride, strontium fluoride and lithium fluoride.  
     
     
         17 . A method as claimed in  claim 15  wherein said low-chlorine content optical fluoride feedstock is selected from the group consisting of a synthetic powder and a pre-melted fluoride crystal material.  
     
     
         18 . A method of making an optical calcium fluoride crystal, including blanks and elements made therefrom, for transmitting below 200 nm wavelengths, said method including: 
 providing a controlled atmosphere optical fluoride crystal furnace for heating an optical fluoride crystal material, said furnace containing a calcium fluoride crystal material and a purified graphite having a chlorine content less than 0.3 ppm Cl by weight,    heating said calcium fluoride crystal material in said furnace containing said graphite having a chlorine content less than 0.3 ppm Cl to provide a scatter-free optical calcium fluoride crystal having a chlorine concentration less than 0.25 ppm Cl by weight and a below 200 nm transmission >99%/cm.